Unless you were on extremely old firmware (>2years) or something else changed in you internet setup regarding ipv6 (even from your isp) there shouldn't be any such issue. Though I could be wrong, I don't think the k addon ever supported the discovery from
https://ipv6-api.hdhomerun.com/discover when they separated such over 2 years ago and that would change with firmware from the past few years or if they now are connecting to SD through ipv6 (primes and legacy equipment still connect through ipv4 as they don't support ipv6, others will only show up at the ipv4 site (which the older links such as my.hdhomerun.com/discover not specifying ip version redirect to)
Also such discovery requires client device and hdhr connecting to SD from same public ip address (if outside your home your client must do internet requests through the vpn connection to your home network so the request for http discovery comes from the same place. Other changes such as NAT by your isp, or vpn connection to internet may also affect such. What client and vpn are you using to connect (i.e. Android typically won't use correct mode to allow local broadcast discovery, and many others simply won't pass such without configuration, but the addon can be set to use http discovery provided the request to SD is from the same public ip).
Another possible change specific to latest firmware if upnp discovery is used (which I dont believe the addon uses) is that only the primary ipv4 and ipv6 addresses get advertised so the link local ones don't and perhaps you had something that those link local addresses were needed.